fix formatting of 2.15.6 release notes
fix formatting of 2.15.6 release notes

thenewguy/wagtail,wagtail/wagtail,zerolab/wagtail,thenewguy/wagtail,wagtail/wagtail,rsalmaso/wagtail,wagtail/wagtail,thenewguy/wagtail,wagtail/wagtail,zerolab/wagtail,zerolab/wagtail,thenewguy/wagtail,rsalmaso/wagtail,rsalmaso/wagtail,rsalmaso/wagtail,zerolab/wagtail,zerolab/wagtail,rsalmaso/wagtail,thenewguy/wagtail,wagtail/wagtail